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Main steam control valves in power plants are required to operate under wide ranges of valve openings and pressure ratios. In the present paper, experimental and numerical investigations are conducted with rigid and flexible valve head supports in order to clarify mechanisms of valve head vibrations caused by unsteady flows around the valve. Results obtained with the rigid support without valve head vibration show that the unsteady flow around the valve head causes pressure fluctuations on the valve head surface with random and impulsive wave forms. With flexible support, the valve head vibrates at near the natural frequency of the valve head support system and the vibrations are excited around the operation condition where the pressure fluctuation becomes large with rigid support valve head. When the valve head vibration becomes large, the pressure fluctuation becomes periodic with the same frequency of the valve head vibration. The numerical result shows that the response of the separated jet delays behind the valve head motion. As a result, the lateral fluid force fluctuations on the valve head provide negative damping on the vibration.

Steam control valves in power plants are operated under wide range of valve openings and pressure ratios during startup and shutdown transients of the plants. It has been clarified by several researchers that transonic flows and flow fluctuations occur on these valves at a certain range of the operation condition. The transonic flow fluctuation can cause intense pressure fluctuation in the valve and in associating piping. The pressure fluctuation on the valve head yields unsteady fluid forces which can cause forced or self-excited vibrations. In the present study, measurements of the fluid forces are carried out. Using a fixed and misaligned valve, the correlation between the fluid forces and the pressure fluctuations on the valve head surface is confirmed. The time mean fluid force acts as restoring force against the valve head misalignment. Using a valve head exciter, unsteady fluid forces associated with the valve head vibration are measured. The result shows that the unsteady fluid force can provide negative damping force which causes the self-excited vibration of the valve head.

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